1. Field of the Invention
The present invention relates to the use of carboxy-modified elastomers as a compatibilizing component in blends containing a halogenated copolymer of a C.sub.4 to C.sub.7 isomonoolefin and a para-alkylstyrene.
2. Description of Related Art
Halogenated copolymers of isobutylene and up to about 4 mole % of isoprene (butyl rubber) are well known polymer materials whose vulcanizates offer some outstanding properties not possessed by many other diolefin-based elastomers. Articles prepared from many cured halogenated elastomers offer improved resistance to oils and greases as well as resistance to oxygen and ozone degradation. Butyl rubber vulcanizates exhibit good abrasion resistance, excellent impermeability to air, water vapor and many organic solvents as well as resistance to aging and sunlight. These properties render these materials ideal candidates for one or more applications such as water hoses, organic fluid hoses, components in tire construction, gaskets, air springs, adhesive compositions and various molded articles.
More recently, a new class of halogenated elastomeric interpolymers have been discovered which offer many of the same properties as halogenated butyl rubber, but are even more ozone and solvent resistant and are more readily curable. These materials are the halogenation product of random copolymers of a C.sub.4 to C.sub.7 isoolefin, such as isobutylene, and a para-alkyl styrene comonomer, preferably containing at least about 80%, more preferably at least about 90% by weight of the para isomer, and wherein at least some of the alkyl substituent groups present in the styrene monomer units contain halogen. Preferred materials may be characterized as isobutylene interpolymers containing the following monomer units randomly spaced along the polymer chain: ##STR1## wherein R and R' are independently hydrogen, lower alkyl, preferably C.sub.1 to C.sub.4 alkyl and X is bromine or chlorine, and wherein the interpolymer is otherwise substantially free of ring halogen or halogen in the polymer backbone chain. Preferably R and R' are each hydrogen. Up to about 60 mole % of the para-alkyl styrene present in the interpolymer structure may be the halogenated structure (2) above.
Most useful of such materials are elastomeric copolymers of isobutylene and para-methylstyrene containing from about 0.5 to about 20 mole % para-methyl styrene wherein up to about 60 mole % of the methyl substituent groups present on the aromatic ring contain a bromine or chlorine atom, preferably a bromine atom. These copolymers have a substantially homogeneous compositional distribution such that at least 95% by weight of the polymer has a para-alkylstyrene content within 10% of the average para-alkylstyrene content of the polymer. They are also characterized by a very narrow molecular weight distribution (Mw/Mn) of less than about 5, more preferably less than about 2.5, a preferred viscosity average molecular weight in the range of from about 300,000 up to about 2,000,000, and a preferred number average molecular weight in the range of from about 25,000 to about 500,000.
These copolymers may be prepared by slurry polymerization of the monomer mixture using a Lewis Acid catalyst, followed by halogenation, preferably bromination, in solution in the presence of halogen and a radical initiator such as heat and/or light and/or a chemical initiator.
Preferred brominated copolymers generally contain from about 0.1 to about 2 mole % of bromomethyl groups, most of which is monobromomethyl, with less than 0.05 mole % dibromomethyl substituents present in the copolymer. These copolymers, their method of preparation, their method of cure and graft or functionalized polymers derived therefrom are more particularly disclosed in U.S. Pat. No. 5,162,445, the complete disclosure of which is incorporated herein by reference.
The aromatic halomethyl groups present in such copolymers permit facile cross linking to be accomplished in a variety of ways, including by means of zinc oxide or promoted zinc oxide curing systems normally used to cure halogenated butyl rubber.
As stated above, the superior properties of vulcanizates based on halogenated isobutylene/para-methyl styrene copolymers (hereafter referred to as HI-PMS) render them eminently suitable in applications where good heat aging, weatherability, ozone resistance, impermeability to liquids, gases and vapors, energy absorption, flex cracking resistance and chemical resistance are important. Such applications include belts and hoses for water or organic fluids, gaskets, components in tire construction, adhesives, various molded articles, conveyor belts, air springs, and the like.
Yet another application suggested for HI-PMS is its use as a blend component with one or more dissimilar elastomers which lack one or more of these properties in order to enhance these properties in compositions containing such dissimilar elastomers and/or to impart one or more beneficial properties of the dissimilar elastomer into compositions containing HI-PMS.
For example, Neoprene (polychloroprene) rubber (CR) has been the material of choice in most power transmission belts, due to its unique combination of properties: oil resistance, toughness, dynamic flex life, good adhesion to other materials and heat resistance up to 100.degree. C. In the past, CR belts have kept pace with the needs of the automotive industry, but recently there is a need for new materials for more demanding applications. First of all, CR belts are encountering greater heat duress in service due to increasing underhood temperatures (up to 150.degree. C.). Secondly, to meet automotive industry's longer warranty periods ("100,000 mile target"), the CR belts must have a lower failure rate with high mean life, even when high temperatures are not encountered. To meet these emerging needs, improvements in heat, ozone, and cut growth resistance of Neoprene belts are desirable.
Nitrile rubber (NBR) is used in automobiles because of its resistance to fuels, a variety of oils and other fluids over a wide range of temperatures. However, nitrile rubber, as such, cannot be used in specific applications requiring heat and ozone resistance. The poor ozone resistance and heat ageing properties of NBR (which is a random copolymer of acrylonitrile and butadiene) are believed to be the result of unsaturation in the backbone of the polymer which permits scission of the polymer chain to occur under certain adverse conditions.
More highly unsaturated rubbers such as natural rubber, polyisoprene, polybutadiene and butadiene/styrene copolymer rubber may exhibit good properties in terms of wear resistance, flexibility, road adhesion and the like, but these materials are also subject to chemical attack and oxygen and ozone degradation, which may limit the useful lifetime of articles prepared from their vulcanizates such as tires, hoses, windshield wipers, gaskets and molded automotive components.
However, the use of butyl, halobutyl or HI-PMS rubber in blends with other elastomers is often limited to those other elastomers which have a mutual compatibility and comparable cure rate behavior with respect to the isobutylene rubber. Thus whereas highly unsaturated elastomers such as polybutadiene or polyisoprene may, in some cases, be reasonably compatible with isobutylene rubber and may be co-vulcanized because of the high availability of sites of ethylenic unsaturation, other elastomers such as polychloroprene, butadiene/acrylonitrile copolymers and like materials containing polar groups along the chain and/or a relatively low degree of ethylenic unsaturation are not so readily co-vulcanized. In the case of blends with these latter elastomers, chemical and ozone resistance may be improved due to the influence of the isobutylene rubber, but often at the expense of a lowering of physical properties such as tensile strength, elongation, modulus and/or abrasion resistance of the co-vulcanizate as compared with the cured elastomer itself.
Furthermore, many rubber compounds contain carbon black as a filler to increase strength, rigidity and other factors. Accordingly, a rubber blend must also be able to incorporate carbon black to be of use in the automotive industry. However, for blends of dissimilar elastomers, problems can arise in achieving optimum carbon black distribution between the microphases of the final product. In blends of elastomers that differ significantly in terms of unsaturation, polarity or viscosity, carbon black tends to locate preferentially in the higher unsaturation, more highly polar or lower viscosity phase.
The role of a compatibilizer in an elastomer blend is manifold: (1) reduce the interfacial energy between the phases, (2) permit a finer dispersion during mixing, (3) provide a measure of stability against gross segregation, and (4) result in improved interfacial adhesion (G. E. Molau, in "Block Copolymers", Ed by S. L. Agarwal, Plenum, New York, 1970, p. 79).
Two elastomers form a compatible mixture when they have at least one of the following characteristics:
Segmental structural identity. For example, a graft or block copolymer of butadiene and styrene is compatible with either polybutadiene or polystyrene. PA1 Miscibility or partial miscibility with each other. Solubility parameter difference &lt;1, generally &lt;0.2 units. For example, poly (vinyl chloride), PVC, poly (ethylacrylate), PEA, poly (methylacrylate), PMA, have solubility parameters in the 9.4-9.5 range and form compatible mixtures. Although, the structure of nitrile rubber (NBR) is entirely different from those of PVC, PMA and PEA, it has a similar solubility parameter of 9.5 and is compatible with these three polymers. PA1 Functional groups capable of generating covalent, ionic, donor-acceptor or hydrogen bonds between the polymers. PA1 a) copolymers consisting of isobutylene and a monomer having the structure of formula 2 wherein R" is hydrogen or C.sub.1 to C4 alkyl, e.g., copolymers of isobutylene and a monohalo-substituted para-alkylstyrene; PA1 b) terpolymers comprising isobutylene and a mixture of monomers having the structure of formulas 1 and 2 wherein R" is hydrogen or C.sub.1 to C.sub.4 alkyl, e.g., terpolymers of isobutylene, a para-alkylstyrene and a monohalo-substituted para-alkylstyrene; PA1 c) terpolymers comprising isobutylene and a mixture of monomers having the structure of formula 2 wherein, with respect to a major proportion of the formula 2 monomer, R" is hydrogen or C.sub.1 to C.sub.4 alkyl and, with respect to a minor proportion of said formula 2 monomer, R" is X, e.g. bromine or chlorine, such as terpolymers of isobutylene, a mono-halo substituted para-alkylstyrene and a di-halo substituted para-alkylstyrene; and PA1 d) tetrapolymers comprising isobutylene and a mixture of monomers having the structure of formulas 1 and 2 wherein, with respect to major proportion of the formula 2 monomer, R" is hydrogen or C.sub.1 to C.sub.4 alkyl and, with respect to a minor proportion of said formula 2 monomer, R" is bromine or chlorine, e.g., tetrapolymers of isobutylene, a para-alkylstyrene, a monohalo-substituted para-alkyl styrene and a dihalo-substituted para-alkylstyrene.